The present invention relates to improved methods for the preparation of simple and composite materials by combustion synthesis, as well as to novel phases prepared by the methods of the invention.
Self-propagating high-temperature synthesis (SHS) or combustion synthesis has heretofore been employed for the manufacture of a variety of different materials [Z. A. Munir, "Synthesis of High Temperature Materials by Self-Propagating Combustion Methods," Ceramic Bulletin 67, 342 (1988); Z. A. Munir and U. Anselmi-Tamburini, "Self-Propagating Exothermic Reactions: The Synthesis of High-Temperature Materials by Combustion," Materials Science Reports 3, 277 (1989)]. In the SHS process, the highly exothermic heat of reaction causes the reaction to propagate in the form of a combustion wave through the reactants, converting them into one or more product phases.
Current research in combustion synthesis has for the most part been directed to the synthesis and processing of a variety of simple and complex materials. Such materials include, for example, oxide superconductors, intermetallic compounds, nanoscale particles, composite materials, and functionally gradient materials. Combustion reactions have also been used in conjunction with other processes for preparation of products having desired properties. These include preparation of dense materials through the application of external pressure, application of ceramic and diamond coatings for steel parts, and formation of joints between dissimilar materials.
Theoretical and modeling studies have been directed to investigation of the nature of the combustion wave in self-propagating high-temperature synthesis and the boundaries delineating its stability. Theoretical analyses have also been utilized in experimental studies on the temperature profile of the combustion wave and its significance in the kinetics of these reactions. In recent investigations, the activation energy of the combustion process determined from the temperature profile analysis was in agreement with that calculated from the temperature dependence of the wave velocity, indicating that the reaction is complete within the confines of the wave [S. D. Dunmead et al., "Temperature profile analysis in combustion synthesis: I, Theory and background," J. Am. Ceram. Soc. 75, 175 (1992); S. D. Dunmead et al., "Temperature profile analysis in combustion synthesis: II, Experimental observations," J. Am. Ceram. Soc. 75, 180 (1992)].
The existence and stability of a self-propagating wave are dictated by the thermodynamic and kinetic properties of the combustion reaction. It has been shown empirically that a linear relationship between .DELTA. H.degree..sub.298 /Cp.sub.298 and T.sub.ad exists for a variety of materials (where .DELTA. H.degree..sub.298 is the enthalpy of formation of the product at 298K, Cp.sub.298 is its heat capacity at 298 K., and T.sub.ad is the adiabatic temperature of the combustion reaction). Experimental observations have demonstrated that self-sustaining combustion reactions cannot exist if T.sub.ad is less than or equal to about 1800 K.; this corresponds to a minimum ratio of .DELTA. H.degree..sub.298 /Cp.sub.298 of about 2000 K. Thus, efforts at establishing such reactions in any given marginal system focus on increasing the value of the ratio .DELTA. H.degree./Cp. For a given reaction, this generally implies increasing .DELTA. H.degree. by increasing the temperature of the reactants prior to ignition.
Theoretical interest in the existence and mode of propagation of a combustion wave has a counterpart in applied research, as these considerations also influence the nature and/or the microstructure of the product phases. Methods to activate the combustion process, primarily by pre-heating the reactants, have been commonly utilized to cause a self-propagating reaction in less energetic systems. A modification of this approach is the use of a chemical oven [U. Anselmi-Tamburini and Z. A. Munir, "The propagation of solid-state combustion waves in Ni-Al foils," J. Appl. Phys. 66, 5039 (1989)].
Pursuant to heretofore known methods, a mixture of metal and/or nonmetallic precursor powders of a suitable size is formed in an appropriate overall atomic ratio corresponding to that in the desired final product. The powder mixture is then suitably pressed to form a green body (for example, in the form of a pellet) with an appropriate green density (for example, on the order of about 50-60% for, e.g., titanium and carbon).
The green body is then treated to initiate the self-propagating high-temperature synthesis reaction, under either an inert atmosphere or an atmosphere which provides one or more additional elements to be incorporated into the final product (e.g., a nitrogen atmosphere for preparation of nitrides and carbonitrides). The SHS process may suitably be effected inside a combustion chamber. The reaction is typically initiated by imparting energy to one end of the sample; this is suitably carried out by, e.g., transferring sufficient energy to the sample in the form of heat radiated from a tungsten coil. Alternatively, the necessary energy for initiation of the reaction may be supplied by, e.g., a laser. Upon ignition, a self-propagating reaction wave begins to move down the sample. Upon completion of the traversal of the wave in a relatively short period of time (as little as one minute, depending on sample size), the compacted material has been converted to the desired final product.
Methods of combustion synthesis of products from electrically-conductive precursors in which a current is passed through the sample has been reported [see, e.g., O. Yamada et al., "Self-propagating high-temperature synthesis of the SIC," J. Mater. Res. 1, 275 (1986); V. Y. Belousov et al., "Some relationships governing initiation of self-propagating synthesis in direct electric heating," Sov. Powd. Met. Powd. Ceram. 10(310), 813 (1989)]. In reports from one research group, it was suggested that in the case of Ti+C combustion, the ignition temperature is independent of the imposed electric power and seems to coincide with the melting point of titanium [V. A. Knyazik et al., "Macrokinetics of high temperature titanium interaction with carbon under electrothermal explosion conditions," Combust., Explos. Shock Waves 21, 333 (1985); V. A. Knyazik et al., "Mechanism of combustion in the titanium-carbon system," Dokl. Akad. Nauk SSSR 301, 689 (1988)].
In the above studies, it was explicitly or implicitly assumed that the role of the imposed electric field in combustion is thermal in nature (i.e., that the field causes an increase in the temperature of the reactants through Joule heating). Thus, the process would be equivalent to a conventional pre-heating of the reactants (by, e.g., thermal means) prior to ignition. However, a major difference between the two methods relates to the rate of heating. The rate of heating accompanying the passage of a current is significantly higher than that observed through thermal (usually radiative) means. Rapid heating has the advantage that pre-combustion (diffusional) reactions are suppressed, and thus the mechanism of the combustion reaction is not significantly altered before ignition.
Pursuant to the known methods, however, a high level of current is typically employed to cause ignition of the materials. This results in very high expenditures of energy. In addition, the elevated temperatures resulting from the use of high levels of current often result in thermodynamically favoring undesired side-reaction; thus, at these higher temperatures the final product may differ significantly in composition from the desired end product. Another major disadvantage of this approach is that it is restricted to the use of conducting materials as reactants. Non-conducting reactants cannot support a current and would require exceptionally high voltages to initiate a current flow. In most cases, however, the application of high voltages can lead to a dielectric breakdown, a phenomenon associated with electronic excitations.
In the prior studies discussed above, the field was applied through direct contact with electrodes. In a more recent work, the combustion of Ti+C and FeO+Al was studied inside an induction coil [A. I. Trofimov et al., "Combustion in Condensed Systems in External Electromagnetic Fields," Int. J. Self-Prop. High-Temp. Synth. 1, 67 (1992)]. It was reported that combustion of mixtures of Ti, C, Al and FeO at a relative poured density of 0.25 under the influence of high frequency was characterized by a higher velocity and a higher degree of conversion relative to that conducted in the absence of the field. It is further reported, however, that the effect of the field is negligible in the heat-affected zone (i.e., ahead of the wave) and that ignition takes place at the same temperature (1000.degree. C.) in the presence or absence of the field. This is rather puzzling, since the imposition of an induction field would result in an increase in the temperature of a surface layer of the reactant mixture. Thus, a different temperature profile should be obtained when the field is applied.
The influence of a magnetic field on the combustion reaction between condensed phases is even less understood. In the only reported study [A. K. Kirdyashkin et al., "Effect of a magnetic field on the combustion of heterogeneous systems with condensed reaction products," Combust., Explos. Shock Waves 22, 700 (1986)] the application of a magnetic field was seen to increase the velocity of the combustion wave (by nearly a factor of three) in a ferromagnetic metal-sulfur system and in other metal-metal systems in which one metal is ferromagnetic. However, the enhancement of the combustion process by the application of the magnetic field was more complex, showing a dependence on particle size of the metal and on the overall stoichiometry of the reactants. It was proposed that the application of the field increased the effective thermal conductivity of the reactants by increasing the contacts between particles of the ferromagnetic metal through a field-induced rearrangement. There was no correlation between the Curie temperature of the metals used and the field effect. In other words, the combustion temperatures were above or below the corresponding Curie temperature, even when an effect was observed. This is one of the reasons which prompted the suggestion that the field influences the process prior to the arrival of the combustion wave. Thus, according to these observations the effect of the magnetic field is not unique. In other words, if the application of field does nothing more than enhance contact between reacting particles, then other means (e.g., higher compaction pressure, etc.) could equally well be employed to achieve this goal.
It is therefore apparent that the limited experimental observations heretofore reported in the literature do not provide a clear understanding of the role of externally imposed electromagnetic fields on the process of self-propagating combustion syntheses. It has been variously reported that the imposition of electric fields on combustion synthesis substrates enhances the combustion process, retards the combustion rate, or results in a thermally inactivated process in the presence of a liquid phase. In addition, the influence of the field is suggested to be chemical in nature, either by enhancing the combustion reaction through electronic excitations and subsequent collision (in the case of gas-phase combustion), or through thermal activation by Joule heating prior to combustion (in the case of solid-solid combustion). While any one of such explanations may be plausible, there has heretofore been no direct teaching of how to exploit an applied electric field in combustion synthesis processes. In addition, it has apparently been generally accepted that fields are useful only in conducting materials where a current is established. This current is believed to cause heating and subsequently a reaction.
As was pointed out above, on the basis of experimental observations, it has been suggested that systems with T.sub.ad less than about 1800 K. will not react in a self-propagating manner, unless the reactants are preheated before ignition. Thus, some materials are difficult to obtain by SHS without preheating. This is especially true for materials that have low adiabatic combustion temperatures, such as SiC, B.sub.4 C, WC, MoB, MOB.sub.2, TaSi.sub.2 and composites of x SiC/MoSi.sub.2 (x=4-8) and Al.sub.2 O.sub.3 /SiC. However, preheating the reactants before ignition can complicate the combustion process. The long time needed in preheating results in higher energy consumption and the possible formation of undesirable intermediate phases through diffusional reactions.
It is an object of the present invention to provide improved methods of combustion synthesis which do not suffer from the drawbacks of the heretofore known methods.
It is a further object of the present invention to provide methods employing modest conditions to prepare materials which have heretofore been difficult to synthesize (e.g., cubic BN).